It was an emotional tribute in which the President Obama vowed that Boston would finish the race.

The service was punctuated by a performance from Yo-Yo Ma, who chose the sarabande Bach's Cello Suite No. 5 in C Minor. It was haunting and beautiful, a piece that Ma also played following Sept. 11.

Yo-Yo Ma lives in Cambridge, Massachusetts, across the river from Boston.

He told All Things Considered that he thinks of this piece as "a struggle for hope."

"The falling motion of most of the notes is contradicted by periodic rising notes. In the last bar of the piece," he said in an email, "all but one of the notes rise and I think that represents the strength of the human spirit and, today, the spirit of Boston."